1. Field of the Invention
The present invention relates to finely-scaled (nanoscale) metal/metal composites, and methods for making them. More particularly, the invention relates to making finely-scaled metal/metal composites by a polyol process. These composites may be in the form of powders, films, aggregates, and other desired morphologies.
2. Description of the Related Art
Nanoscale particles have dimensions in the approximate range 1-100 nm. Because of size confinement effects in these particles, and because the high surface-to-volume ratios in these particles (resulting in high surface effect levels), these particles frequently have properties that differ markedly from the properties of identical materials of larger particles.
Several methods exist for the preparation of nanoscale particles, including vapor deposition, solid-state milling, and solution chemistry. Chemical routes for fabricating nanoscale particles have the following attractive features: the ability to tailor design of materials at the molecular level, the ability to control particle size and distribution, the stabilization of particles against undesirable agglomeration, and cost effective production of large batches of many materials.
A chemical process (referred to as the polyol process) for the synthesis of finely dispersed single element metallic particles in the micron and submicron size regime has been developed recently. This process entails dissolving or suspending a metallic compound in a polyol, and taking the solution or suspension to a temperature where the compound is reduced and the metal precipitates out. Size of the particles can be controlled by, inter alia, controlling the reaction temperature, reaction pH, concentration of polyol or precursors, concentration of nucleation agents, and other reaction variables. Several of these factors will affect the nucleation rate. Higher nucleation rates will tend to limit particle size.
Bimetallic and multimetallic nanocomposites are desired for a number of applications, including magnetic recording and GMR applications. Methods are known for making bimetallic and multimetallic composites. For example, one known method entails the preparation of a metastable solid solution (e.g., by vapor deposition) of immiscible elemental metals, and then annealing that solution to cause phase segregation. This yields a nanocomposite with single domain magnetic particles. However, the methods of making these metastable solid solutions are typically costly (e.g., vapor deposition), as is the subsequent annealing step. Moreover, annealing can degrade the properties of many substrates, including Si substrates. Si tends to oxidize under annealing conditions.
Vapor deposition is useful for film deposition, but will not be cost effective for producing bulk powders. Conversely, mechanical alloying is limited to powder production. Accordingly, it is desired to provide a method that is flexible enough to prepare materials of either morphology, i.e., film or powder.
Another method entails using H2 to reduce metal hydroxides. This method also requires reaction temperatures of several hundred degrees Celsius to reduce the hydroxides to their constituent metals.
Another method entails aqueous borohydride reduction of metal salts. Unfortunately, boron was inevitably incorporated as an impurity in these materials.
Accordingly, it is an object of this invention to make nanoscale multi-metal composites, including bimetallic, trimetallic, and polymetallic composites.
It is a further object of this invention to make these composites essentially boron free.
It is a further object of this invention to make these composites with desirable morphology, such as powders and films (including thin films and thin film multilayers).
It is a further object of this invention to make these composites at relatively low temperatures.
It is a further object of this invention to make metal composite powders and films with desirable magnetic properties (e.g., high coercivity, exhibiting GMR, etc.).
It is a further object of this invention to make metal composite films on a variety of substrates.
These and additional objects of the invention are accomplished by the structures and processes hereinafter described.
The present invention is a polyol method for making composite particles of two or more immiscible transition metals, where these particles are of nanoscale dimensions, and where the constituent metals are of essentially nanocrystalline morphology. The method of the invention has the steps of (1) dissolving or suspending two or more precursor metal compounds in an alcohol or polyol (diol, triol, etc.) solution, and (2) taking the solution or suspension to a temperature where the metal compound reduces, causing the first and second metals to form nanoscale composites. The term xe2x80x9cpolyol solutionxe2x80x9d will be used herein to describe solutions that contain either alcohol or polyol.